1. Field of the Invention
The present invention relates to the field of liquid atomization and in particular to the large-scale production of ultra-fine, homogenous liquid droplets or aerosols, emerging with low velocity.
The invention is also related to an apparatus for large-scale production of a mist, consisting of ultra-fine homogenous liquid droplets or aerosols, which employs the above method of liquid atomization.
2. Description of the Prior Art
In the further description the terms atomization and atomizer refer to the process and device, in which is achieved complete destruction of a jet of an incompressible liquid and a mist, consisting of poly-disperse drops is produced. The apparatuses, employing atomization for producing of ultra-fine droplets are known in the art as nebulizers.
There are known various methods and devices for liquid atomization and below are listed those, which have been used as a basis for devising a great variety of atomizers used in industry and described in many literature sources. The known in the art atomization devices include:
1. Centrifugal mechanical nozzles;
2. Pneumatic nozzles;
3. Centrifugal disk atomizers;
4. Ultrasonic atomizers,
The drops, produced by known in the art atomizing devices usually feature a wide size distribution (polidisperse droplets), which practically excludes their applicability in nebulisers, which are dedicated devices for producing of ultra-fine and monodispersed droplets, having narrow size distribution.
Despite there are known some attempts to overcome this major shortcoming, nevertheless, these attempts do not eliminate some other deficiencies inherent to the above-mentioned atomizing devices. Below these deficiencies are listed.
In the case of mechanical nozzles                Necessity in high pressure (50-200 atm) of liquid delivered to the nozzle;        Impossibility to adjust nozzle capacity and to ensure quality of dispersion during spraying;        Small outlet size (about 0.5 mm), which makes it sensitive to contamination by liquid additives and causes rapid clogging thereof;        Wear of the nozzle outlet due to erosion, which changes capacity and dispersity of spray        
2. In the case of pneumatic nozzles:                Necessity in high gas pressure (4-7 atmospheres), which causes higher velocity of emerging droplets;        Danger of contamination due to the small outlet diameter (0.2-0.4 mm), or due to strict tolerance for the dimension of the outlet slit for compressed gas;        High consumption of the compressed gas per mass unit of sprayed liquid;        Impossibility to control droplets size and quantity for a specific nozzle design.        
3. In the case of disk atomizers:                High cost of the spraying device;        Necessity of careful maintenance, including greasing and monitoring of disk condition;        Danger of imbalance due to high disk rotation speed (20,000 rev/min and above) causing thermal expansion of disk material and, as a result, larger outlet dimensions;        High velocity of emerging droplets (140 m/sec and more), resulting in a larger distance of flight and thus larger diameter of spray;        Ventilation effect due to high disk rotation velocity, which creates low pressure above the disk and affects the configuration of the spray and the flight distance of the droplets;        Impossibility to control the droplets size and their amount at a given disk diameter and rotation speed.        
4. In the case of ultrasonic sprayers:                High cost of device;        Low reliability;        Strong dependence on viscosity and surface tension of the sprayed liquid;        Liquid heating, which affects its properties and, therefore, may not always be permitted;        Limited capacity.        
Some other solutions have been developed to improve the monodispersity of atomization achieved in pneumatic sprayers for example by virtue of disposing a filtering element in the path of a high-pressure gas-liquid flow. This filtering element comprises either a set of nets U.S. Pat. No. (4,941,618), (U.S. Pat. No. 5,431,345), or a thick glass filter (U.S. Pat. No. 5,858,313) or tiny balls arranged in a certain pattern (EP 135390).
Nevertheless, all these solutions failed to overcome such disadvantages as contamination, reduced performance, subsequent clogging up of the outlets and an undesirable high velocity of droplets.
In U.S. Pat. No. 4,757,812, the role of a rotating disk sprayer has been considerably amended by ruling out disk rotation. At the same time, the spray formation procedure remained similar to that of rotating sprayers. Due to the use of compressed air, the sprayer of this invention is closer to pneumatic sprayers. Droplet size, achieved in the device, disclosed in the above patent usually amounts to 2-6 μm and more.
According to a paper “The fundamentals of the ultrasonic atomization of medicated solutions”, R.M.G., Annals of allergy, 1968, 591-600, a high-speed air flow was introduced into the vessel to suppress large drops, resulting from the operation of a high frequency ultrasonic sprayer. This air flow pushes the large drops back into the bath, allowing only the fine drops (1.5-3 μm) into the outlet nozzle. However, the amount of fine droplets reduces as compared to their amount in the spray of a high frequency ultrasonic sprayer operating without high-speed air flow.
Thus despite the existence of numerous atomizing devices there is still a need for a new method and device for producing of small droplets, in which the disadvantages of the prior art atomizers are sufficiently reduced or overcome.